The initial objective of this project is to study the multiple functions of two E. coli replication proteins, the rep protein, a DNA unwinding protein, and the single-stranded DNA bindig (SSB) protein. A new phenotype has been found for the prototype ssb-1 mutation. This phenotype, a block in growth of certain phages, will permit direct selection of new ssb mutants when the ssb mutations are isolated in a specific rep starting strain. New ssb mutations will be studied for their effects on cell DNA replication, recombination, repair, mutagenesis, and other cell functions. SSB protein will be isolated from novel ssb mutants for in vitro studies. A new class of rep mutants, affecting cellular rather than phage functions, will be isolated on the basis of the apparent defective interaction of mutant rep protein and mutant SSB protein. The finding of recessive conditional-lethal rep mutations would prove that rep is an essential protein. The effects of these new rep mutations on cell replication and repair functions will be studied. Interaction of SSB with proteins inserted into the cytoplasmic membrane will be examined, and extragenic suppressor mutations of rep and ssb mutations will be isolated, with the aim of identifying new dna genes and of determining which replication proteins interact with rep and SSB. A long range objective is to identify new dna genes whose products affect DNA structure as well as replication, using strains which detect signals caused by DNA defects. It is expected that mechanisms of DNA replication observed in this work will be analogous to the mechanisms that operate in cells of higher organisms, including humans.